1. Field of the Invention
This invention relates to devices formed from a combination of biological and mechanical elements--"bioartifical" devices. In particular, the present invention relates to a bioartificial kidney which comprises a bioartificial filtration device and a bioartificial tubule processing device.
2. Background of the Invention
End stage renal disorder (ESRD) is a common clinical syndrome involving a decline in renal function, either acutely or chronically. The clinical manifestations of this disorder arise from a decrease in the glomerular filtration rate and an inability of the kidney to excrete the toxic metabolic wastes produced by the body. The complete treatment of ESRD is dependent upon the replacement of the filtrative, reabsorptive, homeostatic and endocrine functions of the kidney as an integrated organ structure.
The excretory function of the kidney, the formation of urine, begins in the kidney with filtration of blood at the glomerulus which is a tuft of capillaries. These capillaries invaginate a surrounding capsule called Bowman's capsule where the renal tubule system begins. The structure of the glomerulus is designed to provide efficient ultrafiltration of blood to remove toxic wastes from the circulation and retain important components within the systemic circulation, such as albumin (Brenner and Humes, New Engl. J. Med. 297:148-154, 1977; Brenner et al., New Engl. J. Med. 298:826-833, 1978, both incorporated herein by reference).
The regulatory function of the kidney, especially with regard to fluid and electrolyte homeostasis, is provided by the tubular segments attached to the glomerulus. It is in the renal tubules where processes of osmosis, diffusion as well as active transport all assist in converting glomerular filtrate into urine. The ultrafiltrate emanating from the glomerulus courses along the kidney tubule which reabsorbs fluid and solutes to finely regulate the excretion of various amounts of solutes and water in the final urine. The functional unit of the kidney is, therefore, composed of the filtering unit, the glomerulus, and the regulatory unit, the tubule. Together they form the basic component of the kidney, called the nephron.
To date, the only successful long-term ex vivo replacement therapy for support of renal function is hemodialysis and chronic ambulatory peritoneal dialysis (CAPD) (Iglehart, N. Engl. J. Med. 328:366-371, 1993; Excerpts from United States Renal Data System 1991 Annual Data Report. Am. J. Kidney Diseases 18(5) Supplement 2:21-30, Nov., 1991). Conventional hemodialysis for ESRD mimics to some extent the filtration function of the kidney by circulating a patient's blood through or over a dialysate solution physically separated from the blood by a porous or permeable wall or membrane. The process results in the preferential diffusion of small molecules, such as urea, from the bloodstream into the dialysate solution. Examples of some hemodialyzers and their function are described in U.S. Pat. Nos. 3,370,710; 3,373,876; 3,505.686; 3,704,223; 3,864,259; 3,884,808; 4,176,069 and 4,354,933, all incorporated herein by reference.
Although hemodialysis adequately removes small molecules from the bloodstream, no method has been established which provides for selectively removing or retaining larger molecules. Furthermore, dialysate solutions must be carefully controlled to ensure that their concentrations of biologically essential materials (such as inorganic salts and glucose) are balanced so that these materials which are present in the blood are retained by the blood. There is a strong need for a solution to these severe drawbacks.
The development of synthetic membranes with high hydraulic permeability and solute retention properties in convenient hollow fiber form has promoted ESRD therapy based upon convective hemofiltration rather than diffusive hemodialysis (Colton et al., J. Lab. Clin. Med. 85:355-371, 1975; Henderson et al., J. Lab. Clin. Med. 85:372-391, 1975; all incorporated herein by reference). Removal of uremic toxins by the convective process has several distinct advantages over diffusion. Convection imitates the glomerular process of toxin removal by providing increased clearance or passage of desirable higher molecular weight molecules and the removal of all unwanted solutes (up to a particular molecular weight cutoff) at the same rate.
Although dialysis has dramatically changed the prognosis of renal failure, it is not a complete replacement therapy, since it only provides filtration function (usually on an intermittent basis) and does not replace the homeostatic, regulatory, and endocrine functions of the kidney. Further, because dialysis functions in a nonphysiologic manner, patients with ESRD on dialysis continue to have major medical problems. The current number of patients with ESRD receiving chronic dialytic therapy in the U.S. is approximately 190,000 with the current growth rate of new patients at 8-9%. A long-term replacement therapy which replaces all of the functions of the kidney and which is less costly than current dialysis therapies is desirable.
In designing a better long-term renal replacement therapy, such as an implantable bioartificial kidney, the essential filtration and regulatory functions of kidney tissue must be developed. Some progress towards such a device has been achieved clinically with the use of polysulphone hollow fibers ex vivo which have been demonstrated to maintain the ultrafiltration function of the kidney in humans for several weeks (Kramer et al., Klin Wochenschr 55:1121-1122, 1977; Golper, T. A. Am. J. Kidney Diseases 6:373-381, 1986, both incorporated herein by reference). Limitations of this device include an increased incident of bleeding from internal or external sites of the patient due to the required anticoagulation to maintain hollow fiber patency, diminution of filtration rate due to protein deposition in the fiber over time, and the large amounts of fluid required to replace the ultrafiltrate which is removed from the blood by the filtering device and also contains useful biological material.
The use of endothelial cells seeded on the interior of the fiber conduits and filtration surfaces has been suggested as a means to provide improved long-term compatibility in vivo (Shepard et al., Surgery 99:318-325, 986; Kadletz et al., J. Thorac. Cardiovasc. Surg. 104:73642, 1992; Schneider et al., Surgery 103:456-462, 1988; all incorporated herein by reference). In this regard, endothelial cell seeding of a small caliber vascular prosthesis has been shown experimentally to reduce long-term platelet deposition, thrombus formation and loss of graft patency (Shepard et al., supra). These constructs have been used solely as vascular conduits and not as filtering devices.
An implantable epithelial cell system derived from cells grown as confluent monolayers along the luminal surface of impermeable polymeric hollow fibers has also been described as a first step for tubule functional replacement (Ip and Aebischer, Artificial Organs 13.:58-65, 1989, incorporated herein by reference). Critical to development of functional renal tissue is the isolation and growth in vitro of specific cells from adult kidney which possess stem cell-like characteristics such that they exhibit a high capacity for self renewal and the ability to differentiate under defined conditions into specialized cells having the correct structure and functional components of a physiologic kidney (Hall and Watt, Development 106:619-633, 1989; Potten and Loeffler, Development 110:1001-1020, 1990; Garlick et al., J. Invest. Dermatol. 97(5):824-829, 1991; all incorporated herein by reference). Recently, methodology to isolate and grow renal proximal tubule stem or progenitor cells from adult mammalian kidneys has been demonstrated (Humes and Cieslinski, Exp. Cell Res. 201:8-15, 1992; incorporated herein by reference). Alternatively, renal proximal tubule stem or progenitor cells can be isolated with electromagnetic cell sorting (Whitesides et al., Trends in Biotechnology 1:144-148, 1983; Padmanabhan et al., Analytical Biochem. 170:341-348, 1988; Spangrude et al., Science 241:58-62, 1988) or cell sorting based upon the density of various cell surface membrane proteins, such as integrins (see Jones et al., Cell 73: 713-724, 1993).
Non-serum containing growth conditions were identified which select for proximal tubule cells with a high capacity for self renewal and an ability to differentiate phenotypically, collectively and individually, into proximal tubule structures in collagen gels. Genetic marking of the cells with a recombinant retrovirus containing the lac-Z gene and dilution analysis demonstrated that in vitro tubulogenesis arose from clonal expansion of a single genetically tagged progenitor cell. Thus, a population of proximal tubule cells resides within the adult kidney which exists in a relatively dormant, slowly replicative state, but which retains a rapid potential to proliferate, differentiate and undergo pattern formation to regenerate the proximal tubule epithelium of the lining of the kidney following severe ischemic or toxic injury.
Ex vivo studies on these renal proximal tubule progenitor dells have demonstrated that the growth factors, TGF-.beta. and EGF, along with retinoic acid, can promote differentiation of these cells into renal tubules (Humes and Cieslinski, Exp. Cell Res. 201:8-15, 1992). Thus, a coordinated interplay between growth factors and retinoids appears to be required to induce pattern formation and morphogenesis. In addition, using immunofluorescence microscopy, it has also been demonstrated that retinoic acid induces laminin A and B.sub.1 chain production in these cells and that purified soluble laminin can be completely substituted for retinoic acid in kidney tubulogenesis (Humes and Cielinski, supra). Retinoic acid, as a morphogen, appears to promote pattern formation and differentiation by regulating the production of an extracellular matrix molecule.
Renal tubule epithelial cells have been demonstrated to maintain the differentiated transport functions of the specific nephron segments from which they were derived (Burg et al., Am. J. Physiology 242:C229-C233, 1982; Steele et al., Am. J. Physiology C136-C139, 1986; Amsler et al., Ann. N.Y. Acad. Sci. 456:420-435, 1985; Husted et al., Am. J. Physiology 250:C214-C221, 1986; Bello-Reuss et al., Am. J. Physiology 252:F899-F909, 1987; Blackburn et al., Kidney International 33:508-516, 1988). These references are incorporated herein by reference.
An implantable bioartificial renal device, which can replace renal function and as a result can circumvent the need for long-term dialytic therapy, would substantially benefit patients suffering from ESRD by increasing life expectancy, increasing mobility and flexibility, increasing quality of life, decreasing the risk of infection, and reducing therapy costs.